Concrete is one of the most widely used materials for construction. It is used in the construction of roads, buildings, and numerous other structures.
In an effort to make stronger concrete there is on going research devoted to increasing the mechanical properties of concrete/cement. In this regard, it is known that the addition of nano-silica particles to cement, increases density, reduces porosity, improves the bond between the cement matrix and aggregates, all of which result in a concrete that shows higher compressive and flexural strength.
Early age cracking can be a significant problem in concrete. Volume changes in concrete drive tensile stress development when they are restrained. Accordingly, cracks can develop when the tensile stress exceeds the tensile strength, which is generally only ten percent of the compressive strength. At early stages, the strength is still developing while stresses are generated by volume changes. Controlling the variables that effect volume change can minimize high stresses and cracking.
The volume of concrete begins to change shortly after it is cast. Early volume changes, within twenty-four hours, can influence tensile stress and crack formation in hardened concrete. Chemical shrinkage occurs due to the reduction in absolute volume of solids and liquids in the hydrating paste. Chemical shrinkage continues to occur as long as cement hydrates. After the initial set, the paste resists deformation, causing the formation of voids in the microstructure.
Autogeneous shrinkage is the dimensional change of cement paste, mortar, or concrete caused by chemical shrinkage. When internal relative humidity is reduced below a given threshold, self-desiccation of the paste occurs resulting in a uniform reduction of volume.